Reinforcing fibers such as glass fibers and aramid fibers have been used as reinforcing materials of rubber products such as rubber belts and tires. These rubber products, however, are subjected to bending stress repeatedly and thereby the performance thereof tends to deteriorate due to bending fatigue. As a result, exfoliation tends to occur between the reinforcing fibers and a rubber matrix, or a deterioration in strength tends to occur due to fraying of the reinforcing fibers. Accordingly, the reinforcing fibers to be used for such rubber products are required to have high bending fatigue resistance.
On the other hand, a timing belt that is used for driving a camshaft of an internal combustion engine of an automobile is required to have high dimensional stability to keep suitable timing. Furthermore, rubber belts that are used for auxiliary drive of an injection pump or power transmission in industrial machines are required to have high strength and high elasticity.
In order to fulfill the above-mentioned requirements, reinforcing cords including specific reinforcing fibers have been used conventionally. For instance, high-strength glass fibers and polyparaphenylene terephthalamide fibers (aramid fibers) have been used as the reinforcing fibers. Recently, carbon fibers and fibers made of polyparaphenylene benzobisoxazole also are used. For example, JP8(1996)-174708A proposes carbon fibers to be used as a tension member of a toothed belt. Cords for rubber reinforcement are required to have various characteristics such as high strength, high elasticity, as well as bending flexibility and fraying resistance. However, conventional reinforcing materials including one type of reinforcing fibers used therein have difficulties in achieving a balance between strength and bending resistance. For instance, a reinforcing cord including carbon fibers used as reinforcing fibers has high strength and high elasticity but low bending resistance and therefore has a problem in that its strength tends to deteriorate through bending.